Federal regulators approved AT&T’s $1.2 billion deal to acquire mobile carrier Leap Wireless Thursday afternoon. The companies announced the deal last summer. AT&T paid $15 per share for the carrier, which had been a leader in the prepaid market. Federal Communications Commission officials said they had concerns about the wireless giant buying the smaller carrier, but regulators said those concerns were alleviated somewhat by AT&T’s commitment to build an LTE service in some Leap markets and divest some spectrum.